This invention relates to a process for decomposing an aromatic aldehyde-hydrogen fluoride-boron trifluoride complex in the presence of a specific decomposing agent, whereby the aromatic aldehyde, hydrogen fluoride and boron trifluoride can be obtained separately, and particularly relates to a process for heat-decomposing an aromatic aldehyde-hydrogen fluoride-boron trifluoride complex without causing any change in the quality of the aromatic aldehyde.
It has been known that p-tolualdehyde can be prepared in a high yield and with a high selectivity with a small amount of o-tolualdehyde as a byproduct by reacting toluene with carbon monoxide by using a catalyst consisting of hydrogen fluoride and boron trifluoride. The reaction is one of the two reactions that constitute the process for producing terephthalic acid from toluene; and p-tolualdehyde obtained through the reaction is used as a starting material for organic synthesis of per-p-toluic acid and p-cresol, etc. and for preparation of polymers. Similarly, 2,4-dimethyl benzaldehyde and 2,4,5,-trimethyl benzaldehyde are formed by reacting m-xylene with carbon monoxide and by reacting pseudocumene with carbon monoxide, respectively.
An aromatic aldehyde-hydrogen fluoride-boron trifluoride complex is formed through the above reaction. In attempting to decompose the complex to obtain an aromatic aldehyde, hydrogen fluoride and boron trifluoride separately, a change in quality of the aromatic aldehyde usually occurs in the presence of the hydrogen fluoride and boron trifluordie, because of the strong bonding force between the aromatic aldehyde and the catalyst. The change in quality of the aldehyde in the decomposing column means the loss of the aromatic aldehyde itself. Furthermore, a small amount of water is produced as a result of the reaction causing the change in quality of the aromatic aldehyde. Water so produced not only deactivates some of the catalysts, but also corrodes the apparatus used for carrying out the reaction. It is necessary to avoid any change in quality of the aromatic aldehyde in case of separating the aromatic aldehyde from the complex. Therefore, the production of water should be kept as low as possible, and it is necessary to decompose the aromatic aldehyde-hydrogen fluoride-boron trifluoride complex as rapidly as possible. Also, it is important to select a deomposing column for effectively carrying out gas-liquid contact. It is preferable to contact the complex with the vapour of a decomposing agent by introducing the complex into the column while refluxing the decomposing agent therein.
For example, British Pat. No. 713,335 discloses a method for continuously obtaining hydrogen fluoride, boron trifluoride and an aromatic aldehyde separately by heat-decomposing an aromatic aldehyde-hydrogen fluoride-boron trifluoride complex, while refluxing a diluent selected from the group consisting of toluene and chlorobenzene in the decomposition system. However, this method has the following disadvantages: (a) The undecomposed boron trifluoride in the form of complex remains in the separated aromatic aldehyde, and (b) the reaction vessel must be made of silver, which is expensive, to prevent corrosion of the vessel by the water which is produced as a result of the reaction causing the change in quality of the aromatic aldehyde. Though it is commercially preferable that such decomposition be conducted under superpressure, a particularly striking loss of the aromatic aldehyde results by maintaining the aromatic aldehyde-hydrogen fluoride-boron trifluoride complex under superpressure. Therefore, since the method of the British Patent has these disadvantages, it can not be said to be a suitable one.
In the U.S. Pat. No. 2,534,017 by Gresham et al, a two-step process for decomposing a p-tolualdehyde-hydrogen fluoride-boron trifluoride complex is disclosed. In the Gresham Patent the first step is effected at a reduced pressure in the absence of any diluent. In the Gresham et al, toluene and cetane are used as a diluent.
In the application of Fujiyama et al, Ser. No. 468025 fled May 8, 1974, now U.S. Patent No. 3,962,343, benzene is used as a diluent for decomposing an aromatic aldehyde-hydrogen fluoride-boron trifluoride complex.